1. Field of the Invention
The present invention relates to a charged particle beam apparatus which employs a scanning electron microscope (SEM) to inspect or observe particles and/or defects on a sample surface. More particularly, it relates to a low-voltage scanning electron microscope (LVSEM) for inspecting uninvited particles and/or defects on surfaces of patterned wafers or masks in semiconductor manufacturing industry.
2. Description of the Prior Art
In semiconductor manufacturing industry, uninvited particles sometime appear and remain on surfaces of masks and/or wafers during semiconductor fabrication process for some reasons, and they impact yield to a great degree. To monitor and therefore ensure the yield, optical apparatuses or called optical tools which are typically based on microscopy have been employed to inspect the particles after some fabrication processes because of their high inspection throughputs and good detection efficiencies. As integrations of IC chips are required higher and higher, critical dimensions of patterns on wafers and masks are shrunk, and consequently smaller and smaller particles will become killers in the yield. On development trends, optical tools are losing their abilities to detect such killer particles due to their longer wavelengths compared to particle dimensions.
Theoretically, an electron beam (e-beam) has a relatively shorter wavelength (such as 0.027 nm/2 keV) compared to particle dimensions (down to several nm), and therefore can provide higher detection sensitivity for small particles than an optical beam. Higher detection efficiency comes from higher detection sensitivity. Conventional e-beam apparatuses or called e-beam tools for inspecting defects on wafer/mask, which are based on Low-voltage Scanning Electron Microscopy (LVSEM) with the normal incidence, can therefore directly perform particle inspection but still be criticized in detection sensitivity and throughput.
To get a high detection sensitivity and high inspection throughput in particle inspection, a dark-field e-beam inspection method is proposed in the cross reference. The dark-field e-beam method employs the difference between the irregular scattering on particles and regular scattering on a sample surface due to an illumination of a primary electron (PE) beam. A dark-field backscattered electron (BSE) imaging, which has a high contrast due to the particles, can be obtained by specifically arranging oblique illumination of the PE beam, collection of backscattered electrons (BSEs) and guiding secondary electrons (SEs). FIGS. 1A and 1B schematically illustrate the formation of the dark-field BSE image. In FIG. 1A, a primary electron (PE) beam 1 passes through a hole of an electron detector 3 and illuminates a sample 2 by an oblique incidence. Consequently, BSEs and SEs are generated and emitted from the illuminated area. Most of the BSEs due to the sample surface travel to the reflection side, and most of the BSEs generated due to the particle 21 moves towards the incidence side and are collected by the detector 3. The electrode 4 attracts the SEs 12 to prevent them from being collected by the detector 3. In other words, the detector 3 only detects the dark-field BSEs 11 which comprise a minority of the diffusely scattered BSEs from the sample surface and most of both the diffusely scattered BSEs and the reflection-like scattered BSEs from the particle 21. In FIG. 1B, the detection signal S3 of the detector 3 is increasing from a low value S3_0 to a high value S3_1, as the PE beam 1 approaches the position P1 of the particle 21. The particle 21 will be shown in the scanning image with a dark background and a high contrast and therefore can be detected out easily.
For a sample with surface trenches or called patterned sample, an e-beam apparatus, which comprises only one single-beam unit employing the foregoing dark-field e-beam inspection method (simply expressed as single-beam dark-field unit hereafter), is only adequate to detect particles inside trenches which are orientated almost parallel to the incidence plane (formed with the surface normal) of the PE beam 1. For the sample 2 in
FIG. 2, one single-beam dark-field unit with an oblique incidence in XOZ plane can detect the particle 21-2 inside the trench 22-2 along the X direction, not the particle 21-1 inside the trench 22-1 along the Y direction, as shown in FIGS. 3A˜3C. As the trench 22-1 lowers the particle 21-1 almost below the surface of the sample 2 in FIG. 3A, the PE beam 1 can only hit the top of the particle 21-1 and consequently the dark-field BSEs 11 disappear. In FIG. 3B, the trench 22-1 is so deep that the PE beam 1 can not hit the particle 21-1 at all. In FIG. 3C, although the trench 22-2 is so deep that the particle 21-2 is much lower than the surface of the sample 2, the PE beam 1 can travel along the trench 22-2 and hit the particle 21-2 from bottom to top as usual.
Accordingly, an e-beam apparatus, which can especially inspect small particles on a patterned sample surface with high detection efficiency and high throughput, is needed. In addition, it will benefit the yield management in semiconductor manufacturing to a great degree if the e-beam apparatus can observe or review the detected particles within the same vacuum chamber as well.